In the fabrication of conveyor belts and in the repair of conveyor belts, especially conveyor belts composed of vulcanizable material, it is common practice to provide a belt press having a pair of heated platens between which two ends of the web adapted to form the belt may be pressed together and bonded.
Such press platens can be of rectangular or rhombic configuration and serve to provide the heat and pressure necessary to join two ends of a length of conveyor belt so as to transform this length into an endless conveyor, to join an end formed by rupture of a conveyor belt after appropriate trimming to another trimmed end in effecting repair, or to join the belt to inserts which are intended to replace the damaged part. Such platens can also be used in presses for repair of conveyor belts by applying patches thereto.
Since heated platens for this purpose are subjected to considerable stress, they have been constructed heretofore of any generally planar framework of structural steel shapes, a heating plate disposed on one side of this framework, a cover plate on the opposite side and electrical heating means in spaces formed within the framework between the plates.
The typical electric heating elements used for this purpose are planar arrays of meanders of an electric heating element within a tubular support (tube-shaped heatging element) or an array of negative--thermistor heating plates, the latter automatically providing control of the heat output.
A typical frame structure utilized for this purpose comprises longitudinal and transverse edge members, i.e. four edge members, two of which extend along the longitudinal edges of the frame or platen and two of which extend along the transverse edges of the frame or platen, each longitudinal member adjoining a transverse member at a corner of the platen.
In addition, intermediate members can extend between the transverse members parallel to one another and to the longitudinal members and in mutually equispaced relationship. In cross section, all of the intermediate members are identical and all of the edge members are identical.
The heating plate and the cover plate, which are coextensive with the periphgery of the frame, sandwich the frame between them.
When reference is made to flat heating elements, I intend to so designate heating elements which lie in a plane parallel to the heating plate, referably thereagainst, and are capable of generating a surface heating action.
In conventional press platens for this purpose, the flat heating elements have generally been constituted in the form of a so-called heating tube, generally a tube of ceramic material, lying in plane and formed with spiral or meandering turns or loops and provided internally with an electric resistance heating element. While the most common arrangement of the heating element is in the form of an Archimedean spiral, thereby requiring the turns to pass through cut-outs in the frame members, the looped arrangement has also been utilized.
It has been found that such platens have a nonhomogeneous temperature distribution across the heated surface thereof, i.e. the temperature varies significantly over the heated surface .
Furthermore when platens of this type are assembled together with other platens in contiguous relationship this nonhomogeneous temperature distribution is even more pronounced with the greatest deviation from uniformity of temperature distribution being located at the junction.